I've had sex while drunk more times than I can count (I like my liquor – it's a work in progress, haha).
I actually worry a lot more about being the drunk(er) guy in bed, so I always disclose it first verbally -- "I'm smashed, just so you know." I know I'm going to be a little clumsy and goofy, may forget myself and get too rough, may take me twice as long to cum (believe it or not, sometimes that's not a good thing), etc.
Not once have I ever actually felt like I was taken advantage of when I was drunk, though. I'm generally the initiator when I'm like that, and the only thing that has really changed is my level of bravery -- I wanted to do it all along. Then again, I also never become this fall-down, incoherent, sloppy drunk I've seen others turn into. I can always walk, talk, and answer questions, no matter how silly I get. I also don't "black out." I always remember.
I suppose the best answer is, "how drunk is he"? If he's coherent enough to verbally talk about how drunk he is, carry on a basic conversation (even if it gets silly, slurry, and giggly), and still insists that he's down to fuck ... I'd be okay with it. I've been on the other side of the equation many times, and I know very well I'm dead serious when I agree that sex should happen, haha.
I think the "no drunk sex because it's always rape" scary admonition fails to recognize the full spectrum of inebriation (also the fact that a lot of the time, it's mutual drunkenness). It's definitely the right thing to not proceed when you're dealing with people that can NOT answer you back, at least not understandably, can NOT stand up and walk around, and don't seem to be fully aware of where they are and what's going on in general. Again, there is a spectrum of "drunk." It's definitely not cool to play around with someone, even if it was a date, who's in that state. If he's past the point of knowing what's going on, you really are a rapist.
Active participation in the event is probably the best way to tell where your partner is, in terms of cognitive responsibility. If he's just a bit tipsy and slur-goofy, but can talk about his plans for next week, how horny he is, is eager to make out and cuddle and explore and is an ACTIVE participant in the play, and can give you a clear, enthusiastic "yes" ... then go for it. He's still in control of his decisions at that point.